Interferences

I am setting up a drone with FPV via 3G USB modem connected to a Raspberry. The problem is that this modem is creating a lot of interference to my RC receiver, so it becomes quite hard to control the drone. The receiver is a quite old Hitec at 75 MHz with a long wire antenna that I had to roll on itself.

Every one is an exercise in separation and grounding until the problem is solved or you give up. Almost all can be solved.

First separate the modem from the receiver as much has you can. Make sure the wires to the Modem are separate from other wires and don't wrap any single or pairs of wire around others making accidental transformers. Solder a .1μF capacitor across every power line and to the ground plate.

Use a single piece of copper flashing to ground the the plane. A wide thin sheet is ideal as it has very low impedance. Keep all ground connections short and close together. Make sure all wires are twisted about the same rate.

It might help to wrap the modem in box made of thin copper sheet. That works for the radio as well but it may detune some of the frequency circuits on some.

In the worst cases I've used side by side 90 ohm coax cable instead of twisted pair for wires by soldering the shields together at each end and every 2 inches. Solder the coax to the ground sheet where it crosses it. The 2 90 ohm coax line has 45 ohm impedance instead of 72 of the twisted pair.

Separating, grounding, and shielding are about all that can be done at this stage.